1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device, more particularly to an NAND-type flash memory.
2. Description of the Related Art
In recent years, in an NAND-type multivalued flash memory, a threshold-value fluctuation of a read cell is generated by a parasitic capacity between floating gates of a memory cell (readout cell) as a readout object and a memory cell adjacent to this read-out cell, and this has been a serious problem (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2004-192789). This threshold-value fluctuation due to this parasitic capacity is referred to as a proximity effect herein. Since threshold values of bit data are tightly distributed in the multivalued memory, there is little margin in a breadth of the threshold-value distribution due to the proximity effect. Additionally, in recent years, a value of the proximity effect itself is increasing owing to miniaturization of the memory cell, and this has been a large barrier to realization of a minute multivalued memory.